U.S. patent number 6,198,864 [Application Number 09/199,988] was granted by the patent office on 2001-03-06 for optical wavelength demultiplexer.
This patent grant is currently assigned to Agilent Technologies, Inc.. Invention is credited to Lewis B. Aronson, Brian E. Lemoff.
United States Patent |
6,198,864 |
Lemoff , et al. |
March 6, 2001 |
Optical wavelength demultiplexer
Abstract
A demultiplexer includes a unitary optically transparent
structure that utilizes focusing relay mirrors to relay a
multi-wavelength beam of light among a series of
wavelength-specific interference filters, with each filter
separating out a specific wavelength component from the
multi-wavelength beam. The relay mirrors are focusing mirrors, so
that the demultiplexer can be operated with a non-collimated light
beam in a manner that controls the potentially large angle of
divergence of non-collimated light, while taking advantage of the
small beam diameter in order to create a demultiplexer with greater
miniaturization.
Inventors: |
Lemoff; Brian E. (Union City,
CA), Aronson; Lewis B. (Los Altos, CA) |
Assignee: |
Agilent Technologies, Inc.
(Palo Alto, CA)
|
Family
ID: |
22739857 |
Appl.
No.: |
09/199,988 |
Filed: |
November 24, 1998 |
Current U.S.
Class: |
385/47; 385/24;
385/37; 398/9 |
Current CPC
Class: |
G02B
6/29367 (20130101); G02B 6/2938 (20130101); G02B
6/4215 (20130101); G02B 6/4214 (20130101) |
Current International
Class: |
G02B
6/34 (20060101); G02B 6/42 (20060101); G02B
006/26 (); G02B 006/42 () |
Field of
Search: |
;385/47,24,37,50
;359/124,127,130 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Nguyen; Thong
Assistant Examiner: Boutsikaris; Leo
Claims
What is claimed:
1. An optical demultiplexer comprising:
a body;
a plurality of wavelength-specific filters connected to an output
region of said body;
input means, integrated into said body, for directing a beam of
optical energy that includes multiple wavelengths toward a first
one of said wavelength-specific filters; and
a relay arrangement of reflective focusing lenses configured to
recurringly converge said beam and to redirect said beam to said
wavelength-specific filters, said reflective focusing lenses being
integrally formed into said body and being positioned relative to
said wavelength-specific filters such that beam convergence occurs
with approach to each said wavelength-specific filter and such that
at least a portion of said beam impinges multiple
wavelength-specific filters and reflective focusing lenses.
2. The optical demultiplexer of claim 1 further including a
plurality of collectors connected at said output region of said
body in a one-to-one correspondence with said plurality of
wavelength-specific filters, each said collector being positioned
to receive optical energy that has passed through a corresponding
one of said wavelength-specific filters.
3. The optical demultiplexer of claim 1 wherein said input means
includes a reflector formed by a surface variation of said body and
positioned to receive said beam and to direct and focus said beam
toward said first one of said wavelength-specific filters.
4. The optical demultiplexer of claim 3 wherein said
wavelength-specific filters transmit optical energy within one
wavelength range and reflect optical energy within other wavelength
ranges.
5. The optical demultiplexer of claim 1 wherein said reflective
focusing lenses are concave aspherical mirrors formed in a linear
array on a surface of said body.
6. The optical demultiplexer of claim 5 wherein said plurality of
mirrors have reflective surfaces oriented to reflect beams of
optical energy toward said plurality of filters such that said
beams contact said filters at the same angle of incidence.
7. The optical demultiplexer of claim 1 wherein said reflective
focusing lenses are ellipsoidal mirrors formed by surface
variations along said body.
8. An optical demultiplexer comprising:
a main optical block having an input region for receiving a beam of
multi-wavelength optical energy and a plurality of outputs for
outputting a plurality of wavelength-specific optical energy
beams;
a plurality of wavelength-specific filters aligned with said
plurality of outputs and connected to said main optical block such
that a first of said filters is impinged by said received beam of
multi-wavelength optical energy, each of said wavelength-specific
filters having optical characteristics that cause transmission of
optical energy at a first set of wavelengths and reflection of
optical energy at a second set of wavelengths outside of said first
set; and
a plurality of converging reflectors formed at a surface of said
main optical block, each said converging reflector being configured
to focus light reflected therefrom, said converging reflectors
being located relative to said wavelength-specific filters such
that each converging reflector receives at least a portion of said
beam of optical energy from one of said wavelength-specific filters
and redirects said received optical energy into a converging
non-collimated beam toward a different one of said
wavelength-specific filters.
9. The optical demultiplexer of claim 8 further comprising an input
reflector located relative to said input such that said beam of
multi-wavelength optical energy is reflected from said input
reflector to said first of said wavelength-specific filters.
10. The optical demultiplexer of claim 9 wherein said input
reflector is a concave reflector that focuses said beam from said
input reflector to said first of said wavelength-specific
filters.
11. The optical demultiplexer of claim 9 wherein said input
reflector is an ellipsoidal reflector with a first focus point at
said input and a second focus point at said first of said plurality
of wavelength-specific filters.
12. The optical demultiplexer of claim 9 wherein said input
reflector is formed at said surface of said main optical block,
said main optical block being a unitary injection molded plastic
body that includes said input reflector and said converging
reflectors.
13. The optical demultiplexer of claim 8 wherein said main optical
block is coupled to a lens array that receives said
wavelength-specific optical energy beams from said plurality of
outputs and that directs and focuses said wavelength-specific
optical energy beams to collectors.
14. The optical demultiplexer of claim 13 wherein said collectors
are optical fibers.
15. The optical demultiplexer of claim 13 wherein said collectors
are optical detectors.
16. The optical demultiplexer of claim 8 wherein said converging
reflectors are concave aspherical mirrors.
17. The optical demultiplexer of claim 8 wherein said converging
reflectors are diffractive mirrors.
18. The optical demultiplexer of claim 8 wherein said input is a
lens formed at said surface of said main optical block such that
said beam of multi-wavelength optical energy undergoes focusing
upon entering said main optical block, said main optical block
being an injection molded plastic piece.
19. An optical demultiplexer comprising:
a monolithic main optical block having an input for receiving a
non-collimated beam of multi-wavelength optical energy;
a plurality of wavelength-selective filters connected to said
monolithic main optical block, each said filter passing
non-collimated light within a first set of wavelengths and
reflecting non-collimated light with a second set of
wavelengths;
means, integrated as a first region of said main optical block, for
focusing said received non-collimated beam and for directing said
received non-collimated beam to a first filter of said plurality of
wavelength-selective filters; and
a plurality of focusing reflectors integrated as a second region of
said main optical block such that at least a portion of said
non-collimated beam is reflected and focused to each one of said
filters, said non-collimated beam of multi-wavelength optical
energy thereby being separated by said plurality of
wavelength-selective filters into a plurality of
wavelength-specific optical energy beams.
20. The optical demultiplexer of claim 19 wherein:
said plurality of focusing reflectors are located in general
alignment along a first plane;
said plurality of wavelength-selective filters are located in
general alignment along a second plane; and
said first plane of said focusing reflectors is parallel to said
second plane of said wavelength-selective mirrors.
Description
TECHNICAL FIELD
The invention relates generally to wavelength division multiplexed
optical communication systems and more particularly to an optical
wavelength division demultiplexer.
DESCRIPTION OF THE RELATED ART
In a wavelength division multiplexed (WDM) optical system, light
from several lasers, each having a different central wavelength, is
combined into a single beam that is introduced into an optical
fiber. Each wavelength is associated with an independent data
signal through the fiber. At the exit end of the fiber, a
demultiplexer is used to separate the beam by wavelength into the
independent signals. In this way, the data transmission capacity of
a fiber is increased by a factor equal to the number of
single-wavelength signals combined into a single fiber.
Many examples of prior art optical demultiplexers exist. One
example of a bulk optical filter-based demultiplexer is disclosed
in U.S. Pat. No. 5,808,763, entitled "Optical Demultiplexer,"
issued to Duck et al. (hereinafter Duck). The optical demultiplexer
of Duck receives collimated light into a glass block and directs
the collimated light onto a single interference filter. The single
interference filter has a wavelength filtering characteristic that
is dependent upon the angle of incidence with which the collimated
light impacts the filter. To manipulate the angle of incidence of
the collimated light impacting the filter, a series of reflective
surfaces are located opposite the filter and are angled such that
the collimated light zigzags between the filter and the reflective
surfaces within the glass block, reaching the filter each time at a
different angle of incidence. The different angles of incidence are
predetermined to enable separation of a multi-wavelength beam of
light into its wavelength components. Although the demultiplexer of
Duck works well for its intended purpose, the beam diameters
involved with collimated light place physical constraints on the
degree of miniaturization that can be achieved in a demultiplexer
of this type. In addition, the reflective surfaces must be
precisely angled to achieve light filtration at the desired
wavelength.
In another known bulk optical filter-based demultiplexer, a
compound objective lens collimates light from an optical fiber and
then directs the light onto a succession of wavelength-specific
optical filters at a particular angle. At each optical filter,
light of one wavelength, or a group of wavelengths, is transmitted
while light of the remaining wavelengths is reflected. The
transmitted light from each optical filter is refocused by
filter-specific compound objective lenses and coupled into outgoing
fibers for subsequent use. The light reflected from each optical
filter propagates back and forth between successive
wavelength-specific optical filters in a zigzag fashion within the
body of the demultiplexer. Although the demultiplexer works well
for its intended purpose, the demultiplexer requires several
discrete objective lenses which must be assembled and precisely
aligned with respect to one another. In addition, as with the Duck
demultiplexer, the use of collimated light limits the degree of
miniaturization that can be achieved in a demultiplexer of this
type.
Another example of a prior art demultiplexer is disclosed in U.S.
Pat. No. 4,675,860, entitled "Compact Wavelength
Multiplexer-Demultiplexer with Variable Filtration," issued to
Laude et al. (hereinafter Laude). Laude discloses a demultiplexer
that utilizes a number of spherical interference filters that are
arranged in series along an optical path of a beam of light that is
emitted from an optical fiber. Each filter is selective to a
particular wavelength and reflects light of the particular
wavelength back to a wavelength-specific output fiber, while the
light of other wavelengths is passed onto the next filter in the
series. While the demultiplexer works well for its intended
purpose, since the filters are located in series along the
direction of light propagation, light of a wavelength that is not
initially reflected by a first filter will pass through each filter
twice. For example, in a three-channel demultiplexer, portions of
the original light beam must pass forwardly and rearwardly through
two filters. In addition, since the filters refocus the diverging
light upon reflection, the curvature of the filters must be
precisely formed. Further, because the filters are arranged in a
series along the optical path, the filters must be bonded to a
device that is formed by combining multiple separately fabricated
parts.
In view of the size constraints involved with bulk optical
multiplexers and the drawbacks involved with utilizing filters
arranged in series along an optical path, what is needed is an
optical demultiplexer that can be easily produced with greater
miniaturization.
SUMMARY OF THE INVENTION
A demultiplexer in accordance with the invention includes an
optically transparent structure that utilizes focusing relay
mirrors to relay a multi-wavelength light beam among laterally
arranged wavelength-specific interference filters, with each filter
separating out a specific wavelength component from the
multi-wavelength beam. The relay mirrors are focusing mirrors, so
that the demultiplexer can be operated with a non-collimated light
beam in a manner that controls the tendency of such a beam to have
a large angle of divergence, while taking advantage of the small
beam diameter in order to create a demultiplexer with greater
miniaturization.
A preferred demultiplexer includes a main optical block,
wavelength-specific interference filters coupled to the main
optical block, and a series of relay mirrors formed within the main
optical block to direct and focus light onto the interference
filters. The preferred main optical block of the demultiplexer is
composed of a monolithic optically transparent material, such as
plastic or glass. Mechanical features at an input end of the main
optical block align and register an optical fiber, so that a beam
of light from the fiber enters the block through a flat input
surface of the block. An objective mirror is integrated into
another surface of the main optical block to receive the beam of
multi-wavelength optical energy from the input fiber and to direct
the beam to the first one of the interference filters. The
objective mirror is preferably a convex surface relative to the
exterior of the block and is shaped such that light that is
incident on the first interference filter has the desired spatial
and angular characteristics. The surface segment that forms the
objective mirror is preferably coated so as to be internally
reflective, however at sufficient angles of incidence, an uncoated
mirror with total internal reflection can be used. In an
alternative embodiment, an objective lens is integrated into the
input surface to focus the incoming beam of light. The focused
light is directed from the objective lens to the first filter by a
flat mirror that is formed on a surface of the main optical block.
In either embodiment, additional internally reflecting surfaces can
be implemented to fold the incoming beam from the optical fiber, so
that the necessary optical distance between the input fiber and the
first interference filter is obtained in a relatively small
space.
The main optical block also includes an output end. The output end
of the main optical block preferably includes a flat output surface
to which the interference filters are attached. In some cases,
mechanical features can be integrated into the output end of the
main optical block to aid in the alignment and registration of the
interference filters.
The interference filters are wavelength-selective filters that are
preferably connected to the output end of the MOB in a linear array
with fixed center-to-center spacing. Each filter has high
transmission and low reflection over a particular range of
wavelengths and low transmission and high reflection over another
range of wavelengths. The preferable transmission spectrum for WDM
demultiplexer applications is a "flat top" shape in which very high
and uniform transmission is achieved over one wavelength range,
while immediately outside the range a very low transmission and
high reflection is achieved. Although the interference filters are
preferably separated into discrete pieces, the interference filters
can also be integrated onto a single substrate that is attached to
the output end of the main optical block. Moreover, the
interference filters can be deposited directly onto the output end
of the main optical block.
The relay mirrors are focusing mirrors that are preferably
integrated into a surface of the main optical block that is
parallel to the surface which is in contact with the interference
filters. The relay mirrors are located and shaped to produce the
desired spatial and angular characteristics of a beam that
propagates in a zigzag fashion between the interference filters and
the relay mirrors. The relay mirrors are preferably convex aspheric
surfaces relative to the exterior of the block and are coated so
that they are internally reflective. In situations in which the
fiber core and the beam diameter at the filters can be treated as
"points," because they are significantly smaller than any other
dimension in the optical system, an ellipsoidal objective mirror
and ellipsoidal relay mirrors with foci at the input fiber and the
filter centers can be utilized to provide more precise imaging.
In an additional aspect of the invention, a lens array is coupled
to the output end of the main optical block to direct and focus the
filtered light that is output from the main optical block to an
adjacent array of detectors. Preferably, the lens array is
integrated into a lens array block that is a structure of plastic
or glass similar to the main optical block. The lens array block
and the main optical block can be made to have complementary
mechanical features that enable the two parts to fit together with
precise alignment. Additional mechanical features on the lens array
block and/or the main optical block can be formed to act as spacers
that fix the distance between the lens array and an optical
detector array or an array of output optical fibers.
When implemented in a preferred four-channel WDM communications
system, light from an optical fiber is coupled directly into the
main optical block through the input surface without the light
being collimated. The objective mirror integrated into the main
optical block focuses and directs the light from the optical fiber
to a first of four wavelength-specific interference filters that
are formed on the output end of the main optical block. The light
propagates from the objective mirror to the first interference
filter at a pre-established angle of incidence. The light that is
transmitted through the first filter is focused by a lens of the
attached lens array onto a detector for signal detection. The light
that is not transmitted through the first filter is reflected from
the first filter and re-reflected to a second filter by the first
relay mirror. The second filter further separates the beam, and the
same process is repeated between the second and third, and between
the third and fourth wavelength-specific interference filters,
using the second and third relay mirrors, respectively. As the
light beam is relayed among the four wavelength-specific filters,
the components of the light beam have been separated by wavelength
and transmitted through the attached lens array to appropriate
detectors of the detector array, thereby demultiplexing the WDM
signal.
Advantages of the invention include the ability to fabricate the
demultiplexer using high volume, low cost techniques, such as
injection molding, without the need to rely on specialized
materials. In addition, because dielectric interference filters
have reflectivities higher than 99% outside of their passband,
essentially all of the light of a particular wavelength reaches the
appropriate filter, thereby limiting signal loss to the
transmission loss of the filter. Because the demultiplexer uses
integrated focusing reflectors in a folded geometry, the
demultiplexer occupies less space than prior designs while
providing the same performance. Further, assembly of the
demultiplexer involves attaching the interference filters to the
main optical block with only a very coarse alignment.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a depiction of a four-channel demultiplexer with an
objective mirror and separate MOB nd lens array in accordance with
the invention.
Fig. 2 is a depiction of a four-channel demultiplexer with an
objective mirror and direct coupling to detectors in accordance
with the invention.
Fig. 3 is a depiction of a four-channel demultiplexer with an
objective lens, an integrated filter array, and a detector array
with an integrated lens array in accordance with the invention.
DETAILED DESCRIPTION
FIG. 1 is a depiction of a demultiplexer 10 in accordance with a
preferred embodiment of the invention. The demultiplexer includes a
main optical block 14 (MOB), an input surface 38, an objective
mirror 40, wavelength-specific dielectric interference filters 20,
22, 24 and 26 coupled to the MOB, and a series of relay mirrors 30,
32, and 36 integrated into the MOB to direct and focus light
between the interference filters. The term "wavelength" is used
herein to designate the approximate center wavelength of a
wavelength distribution that is typical of an optical signal.
In brief, when implemented in a four-channel WDM communications
system, light is coupled from an optical fiber 42 into the MOB
through the input surface 38 without the light being collimated.
The objective mirror 40 integrated into the MOB receives the input
light 44 that is diverging from the input surface and focuses and
directs the light 70 to a first filter 20 of the four
wavelength-specific interference filters that are attached to an
output end 46 of the MOB.
The light beam 58 that is transmitted through the first filter is
focused by an adjacent lens 50 onto a detector 60 for signal
detection. The light beam portion 68 that is not transmitted
through the first filter is reflected by the first filter for
impingement upon a second filter 22 via the first internally
reflective concave relay mirror 30. The second filter further
separates the beam based upon wavelength. The same process is
repeated between the second and third 24 and between the third and
fourth 26 wavelength-specific filters, using the second 32 and
third 36 relay mirrors respectively. By directing the WDM signal to
each of the wavelength-specific filters, the WDM signal is
separated into its four channels. It is important to note that the
relay mirrors are concave mirrors, so that the non-collimated and
diverging light beam is continuously refocused while propagating
between adjacent filters, allowing the demultiplexer to be
extremely small as compared to prior art bulk optical
demultiplexers for non-collimated beams. With the basic structure
and function of a preferred demultiplexer briefly described, more
details of the preferred embodiment and alternative embodiments are
described below.
The MOB 14 of FIG. 1 utilizes a flat input surface 38 at the input
end 39 of the MOB and an objective mirror 40 at another surface of
the MOB to direct the incoming light to the first filter 20. The
objective mirror 40 is preferably a convex mirror (relative to the
exterior of the MOB) integrated into the surface of the MOB in
alignment to receive the multi-wavelength light 44 that is
introduced at the input surface. The objective mirror can be coated
so as to be internally reflective, or at sufficiently high incident
angles an uncoated mirror with totally internal reflection can be
used. As can be seen, the multi-wavelength light 44 is diverging as
it travels from the input surface 38 to the objective mirror.
Ideally, the mirror is configured such that the multi-wavelength
light 70 is converging as the light propagates from the objective
mirror 40 to the first filter. The input structure of the MOB is
designed so that a converging light beam is incident on the first
filter 20 at a non-perpendicular angle with respect to the filters
20-26. For each filter, the preferred angle of incidence relative
to the output end 46 is approximately 78 degrees.
Mechanical features 45 can be formed in the input end 39 of the MOB
14 for aligning and registering the optical fiber 42. The preferred
MOB is composed of a monolithic, homogeneous, and optically
transparent material such as plastic or glass. Injection molding is
preferably used to fabricate the MOB, but other molding techniques
or precision machining techniques can also be used. The dimensions
of the preferred MOB as shown in FIG. 1 may be approximately 7 mm
high, 5 mm long, and 1.75 mm wide, although the exact dimensions
are not critical.
FIG. 2 is an alternative input arrangement in which like elements
are numbered as in FIG. 1. In the arrangement of FIG. 2, the input
fiber 42 is perpendicular to the output end 46 of the MOB. A flat
objective mirror 88 and a convex integrated mirror 41 (concave
relative to incident light) are utilized to direct and focus the
light from the input surface 38 to the first filter 20. FIG. 2 is
one example of a folded geometry that can be implemented, but
various other arrangements are possible. For example, additional
flat reflective surfaces can be used following the input surface to
fold the beam, so that the required optical distance between the
objective and the first filter can be obtained in a relatively
small space.
FIG. 3 is another alternative input arrangement in which like
elements are numbered as in FIGS. 1 and 2. In the arrangement of
FIG. 3, the input surface is an objective lens 82 that is
integrated into the surface of the MOB 14. The objective lens is
preferably an aspheric convex surface that is shaped such that the
light that reaches the first interference filter 20 has the desired
spatial and angular characteristics. Because the light is being
focused by the objective lens, a flat integrated mirror 80 can be
utilized to direct the light from the objective lens to the first
filter. Alternatively, the objective lens can be arranged such that
the light propagates directly from the objective lens to the first
filter. As with FIGS. 1 and 2, mechanical features 43 can be formed
in the MOB to register and align the optical fiber 42 with the
input surface of the MOB.
Referring back to FIG. 1, the output end 46 of the MOB 14
preferably includes a flat output surface. The interference filters
20-26 are preferably attached to or deposited on the flat output
surface. As an alternative, mechanical features can be integrated
into the output end of the MOB to aid in the alignment and
registration of the interference filters. For example, FIG. 2
depicts a design in which depressions are formed in the output end
46 of the MOB to precisely position the four distinct interference
filters 20-26. FIG. 3 depicts another alternative design in which a
single depression is formed in the output end 46 to receive an
integrated filter array 84.
In some embodiments, the interference filters can be deposited
directly onto the output end 46 of the MOB 14. When the filters are
deposited directly onto the output end of the MOB, the output
surfaces do not have to be flat. For example, curved output
surfaces formed at the output end of the MOB may be used to
maintain the desired beam characteristics within the MOB and to
focus the beams of light that are output from the MOB. Although
some examples of the output end arrangement are shown and
described, the exact arrangement is not critical to the invention
and may be modified as would be apparent to one of ordinary skill
in the art.
Referring back to FIG. 1, the interference filters 20-26 are
preferably dielectric filters that are used to separate one optical
wavelength from another. The wavelength-selective filters are
discrete pieces that are preferably arranged in a linear array with
fixed center-to-center spacing. The interface of the interference
filters and the output end 46 of the MOB 14 defines a mirror plane.
Each filter has high transmission and low reflection over a
selected set or sets of wavelengths, called the passband, and low
transmission and high reflection over another set or sets of
wavelengths, called the stopband. The preferable transmission
spectrum for WDM demultiplexer applications is a "flat-top" shape
in which very high and uniform transmission is achieved over the
passband wavelength interval, while immediately outside the
passband very low transmission and high reflection is achieved. The
wavelength interval over which the transmission varies from high to
low should be as small as possible to accommodate narrow channel
spacing. In a preferred four-channel WDM system, the four channels
are centered at approximately 1280, 1300, 1320, and 1340 nanometers
(nm). Another preferred four-channel spacing distribution includes
wavelengths of 820, 835, 850, and 865 nm. Although the
demultiplexer is described with reference to a four-channel
demultiplexer, more or fewer channels can be demultiplexed by
adding or subtracting filters and relay mirrors.
Because the transmission spectrum of an interference filter changes
as a function of the angle of incidence, the best performance is
achieved when the angular extent of the light passing through a
filter is minimized. In addition, the use of materials with high
refractive indices will reduce the dependence of the transmission
spectrum on the incident angle. Although the interference filters
20-26 are separated into discrete pieces in FIGS. 1 and 2, in the
demultiplexer of FIG. 3, the four interference filters 20-26 are
monolithically integrated onto a single substrate to form a filter
array 84 that is attached to the output surface of the MOB. Either
arrangement can be implemented in accordance with the invention. In
another alternative embodiment, the interference filters can be
deposited directly onto the output end of the MOB.
The relay mirrors 30-36 in FIGS. 1, 2, and 3 are converging or
focusing mirrors that are preferably integrated into a surface of
the MOB 14 that is parallel to the faces of the interference
filters 20-26. The relay mirrors are located and shaped to produce
the desired spatial and angular characteristics of a beam that
propagates in a zigzag fashion between the interference filters and
the relay mirrors. The relay mirrors are preferably convex aspheric
surfaces that are coated so that they are internally reflective.
The relay mirrors may also be diffractive mirrors or any other
mirror that causes an increase in the angle of convergence or a
decrease in the angle of divergence of a beam of light.
In the ideal limit, where the fiber core and the beam diameter at
the filters can be treated as points, much smaller than any other
dimension in an optical system, an ellipsoidal objective mirror and
ellipsoidal relay mirrors with foci at the fiber and the filter
centers can be utilized to provide more precise imaging. The
objectives should be chosen such that the ratio of image-to-object
distances gives the angular reduction factor necessary to achieve
satisfactory filter transmission spectra. However, when space
limitations dictate small dimensions for the MOB, this ideal limit
is a poor approximation to the system. In this case, the surface
profiles of the objective mirror or lens and the relay mirrors
should be optimized with a more general aspheric surface to produce
beams with sufficiently small angular and beam diameter at the
filters.
Referring to FIG. 1, in an additional aspect of the invention, an
array of lenses 50, 52, 54, and 56 is coupled to the output end 46
of the MOB 14 to direct and focus the filtered light to the
adjacent array of detectors 60, 62, 64, and 66, or alternatively to
other output devices, such as optical fibers. In the arrangement of
FIG. 1, the lenses 50-56 are integrated into a lens array block 76.
The lens array block is preferably a structure of plastic or glass
similar to the MOB. The lens array block and the MOB can be made to
have complementary mechanical features 72, 74 and 92, 94 that
enable the two parts to fit together with precise alignment. In the
design of FIG. 1, the lens array block and the MOB are fit together
and secured with adhesive. In an alternative to the two-part design
of FIG. 1, a lens array can be integrated into the MOB, with the
MOB having a slot where the interference filters are inserted
between the output end of the MOB and the lens array. In either
case, mechanical features 96 and 98 of the lens array block and/or
the MOB can be formed to act as spacers that fix the distance
between the lenses and the detectors.
The lenses 50-56 integrated into the lens array block 76 are
preferably either piano-convex or bi-convex aspheres. The
piano-convex aspheres have the advantage that the planar sides can
face the interference filters 20-26 with index-matching adhesive
filling in the intervening space to reduce loss due to Fresnel
reflection. The convex side of the lenses can face the detectors
60-66, with the mechanical features 96 and 98 fixing the distance
between the lenses and the detectors. Referring to FIG. 3, in
another embodiment lenses 50-56 are integrated into the substrate
side of a substrate-illuminated detector array 100. In the case of
FIG. 3, a roughly spherical convex surface is etched into the
detector array substrate and the substrate thickness sets the
lens-to-detector distance. Diffractive lenses can also be used for
focusing an output beam and an array of diffractive lenses can be
integrated into the detector substrate, the MOB, or a separate lens
part.
Referring to FIG. 2, in another alternative embodiment, light 110,
112, 114, and 116 output from the interference filters 20-26
propagates directly to the detectors 60-66. In order to implement
direct coupling, the detector array 100 is preferably closely
aligned with the filters and the spatial extent of the beam
incident on the filters is sufficiently smaller than the detector
diameter to allow for alignment tolerances and divergence between
the filters and the detectors. Direct coupling sets a lower bound
on the angular divergence of the beam that is incident on the
interference filter.
As stated above, the light output from the interference filters
20-26 can be directed into optical fibers, instead of detectors.
When outputting into optical fibers, the fiber core diameter and
numerical aperture define limitations regarding the spatial and
angular extent of the output beam. Mechanical features on the MOB
and/or the lens array block can be formed to align and register the
output fibers.
Although the MOB 14 is described with reference to a WDM
demultiplexer, the MOB can be slightly modified and operated in
reverse as a WDM multiplexer. When used as a WDM multiplexer,
single-wavelength beams of light are input to the MOB behind the
interference filters 20-26. The interference filters pass the input
wavelengths and reflect other wavelengths within the MOB. The
combined light is then reflected by the objective mirror 40 into
the optical fiber 42, which acts as an output fiber.
* * * * *